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000019_soo _Tue Nov 24 16:08:25 1992.msg
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Date: Tue, 24 Nov 1992 16:08:25 MST
From: "Michael Soo" <soo>
Message-Id: <199211242308.AA13485@optima.cs.arizona.edu>
Received: by optima.cs.arizona.edu (5.65c/15)
id AA13485; Tue, 24 Nov 1992 16:08:25 MST
To: tsql
Subject: More glossary terms
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\begin{document}
\subsection{Beginning}
\entry{Definition}
The time-line supported by any temporal DBMS is, by necessity, finite
and therefore has a smallest and largest representable chronon.
The distinguished value {\em beginning\/} is a special valid-time event
preceding the smallest chronon on the valid-time line.
Beginning has no transaction-time semantics.
\entry{Alternative Names}
Start, begin, commencement, origin, negative infinity.
\entry{Discussion}
Beginning has the advantage of being intuitive (+E8), and does
not have conflicting meanings (+E5).
``Begin'' appears to be more straight-forward (+E8) but suffers from
conflicting meanings since it is a common programming
language keyword (-E5).
``Start,'' ``commencement,'' and ``origin''
are awkward to use, e.g., ``Start precedes the event,''
``Commencement precedes the event,'' and
``Origin precedes the event.'' (-E8).
Furthermore, choosing start would require us to choose ``end'' for
the opposite concept, and end is a common programming language keyword
(-E5). Origin also has a conflicting meaning relative to calendars (-E5).
Lastly, ``negative infinity'' is longer (-E2) and
slightly misleading since it implies that time
is infinite (-E9). This may or may not be true depending on
theories about the creation of the universe.
Also, negative infinity has a well-established mathematical meaning (-E5).
\subsection{Forever}
\entry{Definition}
The distinguished value {\em forever\/} is a special valid-time event
following the largest chronon on the valid-time line.
Forever has no transaction-time semantics.
\entry{Alternative Names}
Infinity, positive infinity.
\entry{Discussion}
Forever has the advantage of being intuitive (+E8) and does
not have conflicting meanings (+E5).
``Infinity'' and ``positive infinity'' both appear to be more straightforward
but have conflicting mathematical meanings (-E5). Furthermore,
positive infinity is longer and would require us to choose
``negative infinity'' for its opposite (-E2).
\subsection{Initiation}
The distinguished value {\em initiation\/} denotes the transaction-time
when the database was created, i.e., the chronon during which the
first update to the database occurred. Initiation has no valid-time
semantics.
\entry{Alternative Names}
Start, begin, commencement, origin, negative infinity, beginning.
\entry{Discussion}
The arguments against ``start,'' ``begin,'' ``commencement,'' ``origin,''
and ``negative infinity'' are as in the discussion of beginning.
Initiation is preferred over beginning since transaction-time
is distinct from valid-time. Using different terms for the
two concepts avoids conflicting meanings (+E5).
\subsection{Fixed Span}
\entry{Definition}
The duration of a span is either context-dependent or context-independent.
A {\em fixed span\/} has a context-independent duration.
For example, the span {\tt one hour} has a
duration of 60 minutes and is therefore a fixed span.
\entry{Alternative Names}
Constant span.
\entry{Discussion}
Fixed span is short (+E2), precise (+E9), and has no conflicting
meanings (+E5).
``Constant'' appears more precise (+E8) and intuitive (+E9), but
it is also used as a keyword in several programming languages
(-E5).
\subsection{Variable Span}
\entry{Definition}
A span that is not fixed is {\em variable\/}---the value of the span
is dependent on the context in which it appears.
For example, the span {\tt one month} represents a duration
of between twenty-eight and thirty-one days depending on the
context in which it is used.
\entry{Alternative Names}
Moving span.
\entry{Discussion}
Variable span is intuitive (+E9), and precise (+E9).
``Moving span'' is unintuitive (-E9) and has informal spatial connotations
(-E5).
\end{document}